High frequency switching audio amplifier for portable application using 0.35um CMOS technology

Abstract

Recently developed portable devices such as PDAs, MP3 players, and DMB phones employ a power amplifier to drive a small loud speaker. The requirements for this power amplifier are low power consumption, high efficiency, and low distortion. The class-D audio amplifier has been developed to meet these requirements and has been improved significantly owing to advancing technology. While the class-D amplifier satisfies the high efficiency requirement, it requires an output filter for good sound quality. Moreover, it has an EMI problem that is generated by a hard switching noise. These problems adversely affect the applicability of the class-D amplifier to portable devices. In order to make this amplifier suitable for portable applications, the size of its output filter should be reduced. That is, the switching frequency of the class-D amplifier should be raised. However, the conventional class-D amplifier is limited in increasing the switching frequency, as it suffers from both a reverse recovery current loss and the aforementioned EMI problem. In order to overcome these problems, a Δ-Σ modulator power amplifier is proposed in this paper. Utilizing high-speed switching of the Δ-Σ modulator, the size of the output filter is reduced. The switching noise and power loss problems are solved via implementation of soft switching through a zero voltage switching (ZVS) power transfer circuit. In addition, the efficiency of the power amplifier is improved by utilizing an adaptive power control (APC) circuit. The proposed amplifier can operate up to 20MHz switching frequency. The maximum efficiency is 80% with a 4Ω speaker at 1% THD+N, and the lowest THD+N is 0.048%.